This invention relates generally to motorized vehicles, and more particularly to drive system performance control in such vehicles.
Off-highway vehicles, such as mining trucks, are typically provided with a drivetrain in which an internal combustion engine drives a generator that provides electrical current to one or more traction motors. Off highway vehicles typically also utilize dynamic or electric braking (referred to interchangeably herein as “retard effort”), in addition to hydraulically or mechanically operated service friction brakes.
Conventionally, mining trucks run at maximum available power up a grade, However, payload varies significantly from trip to trip, resulting in large variances in on-grade truck speed. This variance tends to cause a line of trucks to bunch up behind the truck with the heaviest load.
In conventional mining trucks, wheel torque is limited to a fixed value which will give acceptable gear life over the overhaul cycle. With limited torque available, a mining truck with a heavy load can also become stuck. A stuck truck is expensive for a mine, as it must be pulled out with a bulldozer, or have its load dumped so the truck can be driven out empty. In either case there is lost production.
Conventionally, the maximum dynamic braking effort is set to a predefined curve as a function of speed. For a specific grade and payload combination there is a maximum speed that the operator can drive and still maintain control of truck speed using only retard effort. This is known as the retard envelope. Once the truck exceeds the retard envelope the driver must use friction brakes to slow the truck back into the retard envelope. On many trucks the friction brakes are dry disks and have limited number of applications. The driver must keep vehicle speed well within the retard envelope to ensure he can maintain control of the vehicle. This limits downhill speed and cycle time.